Add `let_chains` references #1179

c410-f3r · 2022-03-16T11:41:03Z

src/expressions/if-expr.md

nikomatsakis · 2022-03-16T13:26:27Z

c410-f3r · 2022-04-12T12:18:57Z

src/expressions/if-expr.md

-An `if let` expression is equivalent to a [`match` expression] as follows:
-
-<!-- ignore: expansion example -->
-```rust,ignore
-if let PATS = EXPR {
-    /* body */
-} else {
-    /*else */
-}
-```
-
-is equivalent to
-
-<!-- ignore: expansion example -->
-```rust,ignore
-match EXPR {
-    PATS => { /* body */ },
-    _ => { /* else */ },    // () if there is no else
-}
-```
-


It hasn't been the case for quite some time-> rust-lang/rust#80357

Besides, it is strange to document an inner implementation detail

I don't think this is necessarily there as an implementation detail, but as a way to specify how it logically desugars. Then, one can refer to the match documentation to know how things work wrt scoping and temporaries and such. Otherwise, there might need to be some duplication of the documentation.

My mistake, code desugaring only changed for while expressions. for expressions stay the same

ehuss · 2022-04-14T19:30:44Z

src/expressions/if-expr.md

+>  &nbsp;&nbsp; ( [_Expression_]<sub>_except struct expression_</sub>
+> | `let` [_Pattern_] `=` [_Expression_]<sub>_except struct expression_</sub> )


Is there a reason this is surrounded in parentheses?

ehuss · 2022-04-14T19:49:47Z

src/expressions/if-expr.md

-An `if let` expression is equivalent to a [`match` expression] as follows:
-
-<!-- ignore: expansion example -->
-```rust,ignore
-if let PATS = EXPR {
-    /* body */
-} else {
-    /*else */
-}
-```
-
-is equivalent to
-
-<!-- ignore: expansion example -->
-```rust,ignore
-match EXPR {
-    PATS => { /* body */ },
-    _ => { /* else */ },    // () if there is no else
-}
-```
-


I don't think this is necessarily there as an implementation detail, but as a way to specify how it logically desugars. Then, one can refer to the match documentation to know how things work wrt scoping and temporaries and such. Otherwise, there might need to be some duplication of the documentation.

ehuss · 2022-04-14T20:11:48Z

src/expressions/if-expr.md

-if let PAT = EXPR && EXPR { .. }
+## Chains of expressions
+
+It is possible to chain multiple expressions into a single statement to avoid nested declarations.


I think "statement" here is a little confusing. This also doesn't really present any context, or explain how to chain or how chains work. We generally don't use code blocks as a replacement for explaining how something works, but rather as a helpful illustration and example.

Can you try to add some more detail here? I would expect at least a couple paragraphs explaining what chaining is, what the syntax is, what the semantics are, that you can combine boolean values and let pattern bindings, how combining multiple pattern bindings works, etc.

Wrote what was suggested in #1179 (comment)

ehuss · 2022-04-14T20:15:21Z

src/expressions/if-expr.md

@@ -126,30 +104,43 @@ if let E::X(n) | E::Y(n) = v {
 ```

 The expression cannot be a [lazy boolean operator expression][_LazyBooleanOperatorExpression_].
-Use of a lazy boolean operator is ambiguous with a planned feature change of the language (the implementation of if-let chains - see [eRFC 2947][_eRFCIfLetChain_]).


Could this also retain the old note about how to deal with use of lazy boolean expressions when combined with if let? Maybe something like:

The scrutinee expression cannot be a [lazy boolean operator expression] due to ambiguity and precedence with [chains of expressions].
If a lazy boolean operator expression is needed, then parentheses can be used. For example:

if let PAT = ( EXPR && EXPR ) { .. } if let PAT = ( EXPR || EXPR ) { .. }

Done without invalid snippets

ehuss · 2022-04-14T20:21:11Z

src/expressions/if-expr.md

+```rust
+fn single() {
+    let outer_opt = Some(Some(1i32));
+
+    if let Some(inner_opt) = outer_opt
+        && let Some(number) = inner_opt
+        && number == 1
+    {
+        println!("Peek a boo");
+    }
+}


This example isn't super clear to me. Can it perhaps be broken up in two and explain what it is illustrating? Maybe something like this:

Suggested change

```rust

fn single() {

let outer_opt = Some(Some(1i32));

if let Some(inner_opt) = outer_opt

&& let Some(number) = inner_opt

&& number == 1

{

println!("Peek a boo");

}

}

The following is an example of chaining multiple expressions, mixing `let` bindings and boolean expressions, and with expressions able to reference pattern bindings from previous expressions:

```rust

let outer_opt = Some(Some(1i32));

if let Some(inner_opt) = outer_opt

&& let Some(number) = inner_opt

&& number == 1

{

println!("Peek a boo");

}

```

The above is equivalent to the following without using expression chains:

```rust

ehuss · 2022-04-14T20:49:35Z

src/expressions/if-expr.md

 ```

+The only remark is the fact that parenthesis (`if (let Some(a) = opt && (let Some(b) = a)) && b == 1`) and `||` operators (`if let A(x) = e1 || let B(x) = e2`) are not currently supported.


The note about || not being allowed is already stated above, so I don't think it needs to be repeated here.

And the grammar doesn't actually define if (let …) as being valid, so this is a little confusing. I left a comment on the stabilization PR, but it seems like there is some disconnect between what this documentation is stating and how it actually works.

Generally the reference doesn't specify what may or may not be supported in the future, so I'm not sure it is worth stating anything.

ehuss · 2022-04-14T20:55:30Z

src/expressions/if-expr.md

-## `if` expressions
+## Syntax
+
+The same syntax is used by `if`, `if let` and chains of expressions.


I was perhaps intending maybe a little bit of a greater rewrite here, as I'm not sure I entirely see if and if let being fundamentally different. Is there a reason to keep them as separate sections?

I think what I was thinking is it would say something like:

The condition operand can be a list of one or more IfLet expressions separated by &&.
If all of the boolean values evaluate to true and all patterns match their scrutinee value, the consequent block is executed and…
…and include a description of pattern bindings, scopes, && priority, etc.

I guess I'm not seeing why they would be kept separate. if EXPR {} just seems to be a special case where there is an if expression with a single boolean value.

I don't have enough free time to perform a greater rewrite

Do you mind if I push some changes to your branch?

ehuss · 2022-07-24T23:46:53Z

src/expressions/if-expr.md

+> &nbsp;&nbsp; `if` _IfLetList_ [_BlockExpression_]\
+> &nbsp;&nbsp; ( `else` _IfLetList_ [_BlockExpression_] )<sup>\?</sup>


This grammar doesn't look correct to me. The else part seems to have multiple issues:

Doesn't allow if foo {} else {} where it is an unconditional else block.

Doesn't allow if foo {} else if bar {} where there is an else if.

Seems to allow if foo {} else bar {} which isn't valid.

ehuss · 2022-07-24T23:47:23Z

src/expressions/if-expr.md

-The expression cannot be a [lazy boolean operator expression][_LazyBooleanOperatorExpression_].
-Use of a lazy boolean operator is ambiguous with a planned feature change of the language (the implementation of if-let chains - see [eRFC 2947][_eRFCIfLetChain_]).
-When lazy boolean operator expression is desired, this can be achieved by using parenthesis as below:
+The expression cannot be a [lazy boolean operator expression][_LazyBooleanOperatorExpression_]. Scrutinee expressions also cannot be a [lazy boolean operator expression] due to ambiguity and precedence with [chains of expressions].


These links seem to be broken.

Also, can you keep it to one sentence per line?

This updates the `if` and `while` expressions so that they each are presented as a single expression kind with multiple condition operators instead of being logically separated from their `let` counterparts. This also includes various fixes and additions.

ehuss

Thanks!

I pushed the changes that I was thinking of in terms of unifying if/if let and while/while let.

Update books ## reference 2 commits in a92be0fef439b3d8e0468d82cb24812d303520a0..f3d3953bf3b158d596c96d55ce5366f9f3f972e9 2022-07-21 19:01:23 -0700 to 2022-08-01 17:17:37 -0700 - Add `let_chains` references (rust-lang/reference#1179) - Remove outdated warning (rust-lang/reference#1243) ## rust-by-example 18 commits in 3155db49b0d57cd82c65456ac210b69ecec5ccb1..ee342dc91e1ba1bb1e1f1318f84bbe3bfac04798 2022-07-05 20:35:53 -0300 to 2022-07-27 11:06:36 -0300 - Closure inferred twice (rust-lang/rust-by-example#1588) - fix a syntax bug in example assembly (rust-lang/rust-by-example#1511) - Minor grammar change in src/std/rc.md paragraph 2 (rust-lang/rust-by-example#1586) - Fix typo in asm.md (rust-lang/rust-by-example#1585) - Fix incorrect padding in fixed-width print (rust-lang/rust-by-example#1584) - Update print.md (rust-lang/rust-by-example#1582) - add-chapter-on-defaults (rust-lang/rust-by-example#1580) - Fix typo (rust-lang/rust-by-example#1579) - fix a compile error (rust-lang/rust-by-example#1578) - Suggest using mod.rs pattern to share test code (rust-lang/rust-by-example#1577) - fix a compile error in iter_any.md (rust-lang/rust-by-example#1576) - Mention attribute like macros in attributes.md (rust-lang/rust-by-example#1574) - Update exercise to be clearer (rust-lang/rust-by-example#1573) - fixes link for turbofish in testcase_mapreduce.md (rust-lang/rust-by-example#1572) - Fix inconsistency between comment and code in hello/print.md (rust-lang/rust-by-example#1571) - Fixes a typo in print.md (rust-lang/rust-by-example#1570) - into_iter-moves-elements (rust-lang/rust-by-example#1569) - Fix a typo in print.md (rust-lang/rust-by-example#1568) ## rustc-dev-guide 16 commits in d5201cddace979b299ec1bf9fd8997338151aa9d..04f3cf0bb2f5a6ee2bfc4b1a6a6cd8c11d1c5531 2022-07-21 04:48:49 +0200 to 2022-07-31 07:46:57 +0200 - address review comment - accept review suggestion - try address review comments - summary of chapter - Update src/building/compiler-documenting.md - revamp doc-build chapter - minor fixes - Prefer relative links - Fix the link to clippy docs - Fix the link to `ResolverAstLowering` - Fix the link to `ProcMacro` trait - Fix the link to `Lazy<T>` - Add instructions to fix build errors in std after adding a new target - Document how to build a cross-compiler - Add documentation about Microsoft provided debuggers and CodeView/PDB… (rust-lang/rustc-dev-guide#1406) - rust-analyzer is now a subtree ## embedded-book 2 commits in 766979590da8100998f0d662499d4a901d8d1640..befe6840874311635c417cf731377f07234ee373 2022-07-04 09:13:58 +0000 to 2022-07-25 07:51:14 +0000 - Updated instructions for running first Hardware example (rust-embedded/book#323) - Improved ligability for hardware.md (rust-embedded/book#324)

c410-f3r commented Mar 16, 2022

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c410-f3r force-pushed the let-chains branch from 514d8b3 to d3cee16 Compare April 12, 2022 12:16

c410-f3r commented Apr 12, 2022

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Stabilize let_chains in Rust 1.64 rust-lang/rust#94927

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ehuss reviewed Apr 14, 2022

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c410-f3r force-pushed the let-chains branch from d3cee16 to 9058c4b Compare July 21, 2022 11:37

ehuss reviewed Jul 24, 2022

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c410-f3r force-pushed the let-chains branch 2 times, most recently from bd2f4b2 to 68f3e4a Compare July 25, 2022 11:14

c410-f3r and others added 2 commits July 25, 2022 15:42

Add let_chains references

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ehuss force-pushed the let-chains branch from 3551c46 to b83d8e6 Compare July 25, 2022 22:42

ehuss approved these changes Aug 2, 2022

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ehuss merged commit f3d3953 into rust-lang:master Aug 2, 2022

ehuss mentioned this pull request Aug 2, 2022

Update books rust-lang/rust#100042

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This was referenced Aug 14, 2022

Revert let_chains stabilization rust-lang/rust#100538

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Revert let chains reference docs #1251

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c410-f3r mentioned this pull request Jun 18, 2024

Add documentation for meta variable expressions #1485

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est31 mentioned this pull request Nov 10, 2024

Stabilize let chains in the 2024 edition rust-lang/rust#132833

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Add `let_chains` references #1179

Add `let_chains` references #1179

c410-f3r commented Mar 16, 2022

nikomatsakis commented Mar 16, 2022

c410-f3r Apr 12, 2022 •

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		>    ( [_Expression_]<sub>_except struct expression_</sub>
		> \| `let` [_Pattern_] `=` [_Expression_]<sub>_except struct expression_</sub> )

		```

		The only remark is the fact that parenthesis (`if (let Some(a) = opt && (let Some(b) = a)) && b == 1`) and `\|\|` operators (`if let A(x) = e1 \|\| let B(x) = e2`) are not currently supported.

		>    `if` _IfLetList_ [_BlockExpression_]\
		>    ( `else` _IfLetList_ [_BlockExpression_] )<sup>\?</sup>

Add let_chains references #1179

Add let_chains references #1179

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c410-f3r commented Mar 16, 2022

nikomatsakis commented Mar 16, 2022

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Add `let_chains` references #1179

Add `let_chains` references #1179

c410-f3r Apr 12, 2022 •

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